This application is a 371 of PCT/EP00/10968, filed on Nov. 1, 2000 and claims the benefit of application EPO 99203616.0, filed on Nov. 2, 1999.
The present invention relates to a process for the purification of industrial waste water originating from a process for producing propylene oxide.
The treatment or purification of waste water originating from industrial processes for producing propylene oxide and containing at least hydrocarbons and salts (organic and/or inorganic) normally is a relatively expensive procedure. Environmental legislation nowadays puts stringent demands on the purification of waste water streams from industrial processes, particularly when the purified waste water is to be released into the environment. Accordingly, the choice of a purification method for industrial waste water is generally bound by practical, environmental and economic considerations.
One industrial process for producing propylene oxide, wherein a relatively large amount of waste water is produced is the styrene monomer/propylene oxide (SM/PO) co-production process. In general such SM/PO process involves the steps of: (i) reacting ethylbenzene with oxygen or air to form ethylbenzene hydroperoxide, (ii) reacting the ethylbenzene hydroperoxide thus obtained with propene in the presence of an epoxidation catalyst to yield propylene oxide and 1-phenyl ethanol, and (iii) converting the 1-phenyl ethanol into styrene by dehydration using a suitable dehydration catalyst. In the last step water is produced. In addition to this reaction water organic by-products such as aliphatic and aromatic hydrocarbons, aldehydes, ketones, alcohols, phenols and organic acids are produced in the course of the entire process. Some of these by-products are separated from the main products with the aid of clean water and the organic acids are neutralized using a basic aqueous solution, such as an aqueous sodium (bi) carbonate and/or sodium hydroxide solution. Furthermore, additional water is introduced with the air in the step (i) and as steam in step (iii) of the above process, while also in other parts of the process some water may be used.
The waste water from an SM/PO production plant typically contains a total of from 1.0 to 3.5 wt % of non-salt organic compounds and from 3.0 to 6.0 wt % of organic salts. It may further contain up to 2.0 wt % of sodium carbonate and sodium bicarbonate and/or traces of sodium hydroxide, depending on the basic solution used in the neutralization of organic acids.
The input of clean water to an SM/PO plant can be up to tens of thousands kg per hour, while the output of waste water is normally about 50% higher than the input of clean water. The waste water cannot be discharged without additional purification treatment. As has already been indicated above, however, the choice of a suitable purification treatment is limited due to all sorts of practical, environmental and economic considerations.
Another well known method for producing propylene oxide, which also produces substantial amounts of waste water, is the co-production of propylene oxide and methyl tert-butyl ether (MTBE) starting from isobutane and propene. This process is well known in the art and involves similar reaction steps as the SM/PO process described above. In the epoxidation step tert-butyl hydroperoxide is reacted with propene forming propylene oxide and tert-butanol. Tert-butanol is subsequently etherified with methanol into MTBE, which is used as an additive in motor fuels.
An example of a process for treating SM/PO waste water is disclosed in U.S. Pat. No. 5,276,235. In this process the waste water is first subjected to a distillation treatment to separate water and light organic material from a concentrated aqueous waste bottom stream, subsequently admixing this bottom stream with aqueous acid, suitably sulphuric acid, then phase separating the resulting mixture into an aqueous sodium-containing phase and an organic phase and finally separately recovering the phases. In U.S. Pat. No. 5,675,055 an improved version of this process is disclosed, wherein the bottom stream recovered from the distillation step is mixed with aqueous acid and a water-immiscible organic solvent before phase separation and recovery of the phases takes place.
It would be useful to provide a method for treating industrial waste water resulting in a purified water stream which is fit for re-use in the process or can be subjected to a subsequent biotreatment resulting in water which is sufficiently pure to meet all requirements as set by environmental legislation for discharge into surface water.
Accordingly, the present invention relates to a process for treating waste water from an industrial process for producing propylene oxide, which process comprises the steps of:
(a) subjecting the waste water to a multi-effect evaporation treatment resulting in a vaporous top fraction and a liquid bottom fraction containing the non-volatile contaminants; and
(b) condensing at least part of the vaporous top fraction into a liquid stream which is subjected to a stripping treatment resulting in an overhead stream containing volatile waste organic material and purified water as the liquid bottom stream.
The purified water obtained is sufficiently pure to be re-used in an industrial process as e.g. cool water, but can also be subjected to a biotreatment resulting in a pure water stream, which should be sufficiently pure for discharge into surface water.